Tap water quality

Thames Water customers were warned in October that bills were likely to rise by as much as £80 a year to help pay for the so-called "super-sewer".

The company also said costs had risen because of higher energy prices and inflation.

Chief executive Martin Baggs said the company had spent £17bn on its infrastructure since it was privatised in 1989.

He said: "As a direct result of this investment, the quality of our customers' tap water and the environmental compliance of our 350 sewage works are better than they've ever been, and leakage from our 20,000 mile network of water mains remains close to its lowest-ever level.

"This transformation would never have been possible under public ownership."